RFR: 8358013: [PPC64] VSX has poor performance on Power8 [v3]

[Martin Doerr]

> Power8 only has limited VSX instructions for the superword optimization and the Vector API and the performance is bad. Let's only use it on Power9 and newer by default. This change excludes the VSX registers from C2 register allocation for Power8. VSX instruction usage gets limited to a few places like intrinsics.
> 
> Note: Power8 is an old processor and performance optimizations for it are no longer planned.

Martin Doerr has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:

 - Merge remote-tracking branch 'origin' into PPC64_disable_SuperwordUseVSX_Power8
 - Improve description of 8358013: [PPC64] VSXSuperwordUseVSX.
 - 8358013: [PPC64] VSX has poor performance on Power8

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Changes:
  - all: https://git.openjdk.org/jdk/pull/25514/files
  - new: https://git.openjdk.org/jdk/pull/25514/files/1f8b0e91..599a4f36

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=25514&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=25514&range=01-02

  Stats: 32865 lines in 385 files changed: 12812 ins; 12713 del; 7340 mod
  Patch: https://git.openjdk.org/jdk/pull/25514.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25514/head:pull/25514

PR: https://git.openjdk.org/jdk/pull/25514